Membrane flow from various parts of the cell surface will be investigated using paramecium caudatum as a model cell system. Endocytosis from the cell surface will be followed with time using pulse-chase studies incorporating electron-microscpic tracers -horseradish peroxidase and cationic ferritin - into the endosomes and polystyrene latex spheres into phagosomes. All sites of endocytic-exocytic membrane flow on the cell surface of P. caudatum can be identified and studied since an alveolar system underlies the plasma membrane thus providing a semi-rigid surface on this cell. We will follow some phases of digestive vacuole membrane--the fate of endosomes which are removed during vacuole condensation and the source of secondary lysosome formation at states of digestion. The fate of the endosomes retrieved from the old digestive vacuole following defecation is already known (Allen & Fok, 1980). We will also investigate two other sources of endosomes at the cell surface, those from parasomal sacs which gives rise to "thick-membraned" vesicles and the "thin-membraned" remnants or cisternae resulting from trichocyst discharge. Using a combination of markers and microscopic monitoring of defecation we outlined a series of studies which allow us to separate and to follow the fate of each endosome population. Thus we will be able to detect the extent of endosome membrane intermixing in the cytoplasmic compartment. Preliminary observations suggest that in some cases membrane is returned to the Golgi apparatus (trichocyst membrane) or to the lysosomal system (endosomes from condensing vacuoles) while in at least one case membrane is shuttled from the cytoproct back to the cytopharynx without passing through the Golgi or lysosomal systems. Basic information on how normal, healthy, model cells reutilize membranes for their best economic advantage will be obtained from this study.